Slipping of the rotor poles in a synchronous machine operating as a part of a power generating or drive unit can occur as a result of a delayed elimination of a fault in the electric power network or as a result of improper excitation of the synchronous machine. In consequence of successive pole slips, a synchronous machine may fall-out of synchronism with the electric power network. During a pole slip in the case of the generator-mode operation of a synchronous machine, oscillations of the rotor torque and fluctuations in active power delivered to the electric power network can occur. If the pole slip results from the loss of excitation, the mechanical and electrical effects can be negligible, but if the phenomenon has occurred with full excitation, then its effects can be harmful for the generator itself and for the electric power network. Moreover, considerable changes in active power during pole slipping are accompanied by voltage fluctuations on the output terminals of the synchronous generator.
A method for measuring the load angle, also known as the power angle, and a method for detecting pole slipping in a synchronous generator, based on the values of the load angle, is known from patent application WO2008/102105. That method is realized by measuring the value of the load angle of the synchronous generator working in a unit containing an exciter and a machine with permanent magnets which excites the exciter, and it is provided with with a device for measuring the voltage induced in the stator of the machine with permanent magnets and a device measuring voltages induced in the stator of the synchronous generator, as well as a device for comparing the measured voltage waveforms. In the presented unit, the load angle is measured by means of comparators and the measurement consists in comparing the phase displacement of the voltage waveforms of the generator stator with the voltage waveform of the rotor of the machine with permanent magnets. In order to make the comparison, both voltages are transformed from sinusoidal waveform to the form of a rectangular signal, and then the phase displacement between them is measured. Next, the load angle is calculated using the values of the phase displacement, and after that the value of the load angle is compared to the reference value which is determined for the synchronous generator, and changes in the load angle over time are calculated. If the value of the load angle exceeds the reference value, this will directly indicate a possible detection of a pole slip in the synchronous generator. If pole slipping is detected, an appropriate alarm signal will be sent to the device that controls the generator to take further measures consisting in opening the generator circuit breaker and instantly switching the generator off. The change in the value of the load angle over time is used as the indicator of the occurrence of pole slipping. If a sudden change in the value of the load angle which is bigger than the predetermined threshold value is detected, an appropriate alarm signal will be sent to the device that controls the operation of the generator and further action will be taken resulting in switching the generator off. The accuracy of the pole slipping phenomenon detection depends directly on the reference value which is determined separately for different types of synchronous generators. The accuracy of load angle measurement depends on the accuracy of instruments used for measuring voltages induced in the stator of the machine with permanent magnets and voltages induced in the stator of the synchronous generator, or the device that compares the measured voltage waveforms. In the presented method, the detection of the slipping phenomenon in a synchronous generator depends on the type of the exciter used for exciting the generator, and in the case of synchronous generators which contain exciting equipment other than that presented in the described method, it cannot be achieved. If there is no voltage signal from the stator of the machine with permanent magnets which excites the exciter, the comparison between such signal with the signal of the voltage waveform of the generator stator cannot be done.
In two publications, first wrote by Redfern M. A., Checksfield M. J.: “A new pole slipping protection algorithm for dispersed storage and generation using the equal area criterion”, IEEE Transaction in Industrial Application, Vol IA-23, No. 5, September 87. pp 777-785. and second wrote by Redfern M. A., Checksfield M. J., H. T. Yip, “Field Trials to Demonstrate the Performance of a new Pole Slipping Protection” Developments in Power System Protection 25-27 Mar. 1997, Conf. Pub, No. 434 pp. 44-47, the method for protecting a synchronous machine against damage caused by rotor pole slipping is presented.
In these publications the power based method of pole slipping of synchronous generators operating in parallel with a utility supply system is presented. The method uses three phase reactive power Q, three phase active power P, and rate dP/dt of change of active power in order to detect operation past the Critical Stability Point. Reactive power Q is used to tell if the generator is operating at load angles of greater than 90′. The reactive power Q trip level, Qtrip is calculated from a quadrature axis synchronous reactance Xq, and is entered as an external setting. The active power trip level Pt is automatically adjusted according to the generator operating point. The rate of change of power trip setting is also continuously adjusted according to the generator operating point. The 1.5 power system cycle time constraint is introduced to ensure that the method remains stable during short circuit faults. The majority of faults will not satisfy all of the trip criteria, but the few that do only cause the criteria to be satisfied for less than one power system cycle. The method uses the generator parameters, the quadrature axis synchronous reactance Xq, the direct axis transient reactance Xd, along with the generator rating Sgen, and generator operating point to derive the trip levels. By dynamically adjusting the trip settings according to the current operating point, the method can use sensitive trip levels to quickly detect pole slips which occur due to steady state or dynamic instability. During transient disturbances, larger trip levels are automatically used, ensuring that there are no false trips during stable power swings.
Similar method is presented in publication Redfern M. A., Checksfield M. J., “A study into a new solution for the problems experienced with pole slipping protection”, IEEE Transaction on Power Delivery, Vol. 10, No. 2, April 98. pp. 394-404. The only difference from the two previous presented publication is in different approach in calculation an equation for Qtrip, where Qtrip is a threshold levels for reactive power Q.